Semiconductor lasers producing visible light have been known for some time. Generally, those lasers employ materials from the InGaAlP system of materials in the various layers within the laser. Typically, the active layer of these lasers is In.sub.0.5 Ga.sub.0.5 P. That material and all active layer materials in semiconductor lasers must have a "direct" transition energy band structure. In these kinds of materials, as illustrated in FIG. 2(a), the distribution of energy states in the conduction band 24 has a minimum 25 that is directly opposite a maximum 26 of the energy state distribution in the valence band 27 when charge carrier energy is plotted as a function of wave vector. In this type of direct transition energy band structure, electrons and holes can directly recombine to radiate light efficiently without interaction with a phonon. In an indirect transition semiconductor material, i.e., when the energy density minimum and maximum are not directly opposite each other in the wave vector graph, the recombination of an electron and a hole requires a phonon interaction. Light emission is not efficient in direct transition semiconductor materials and laser oscillation has not been achieved in them.
Although the minimum conduction band energy and the maximum valence band energy in a direct transition semiconductor material determine the energy band gap and the wavelength of radiated light when that material is used as the active layer in a semiconductor laser, the conduction band edge structure of binary, ternary, and quaternary III-V compound semiconductor materials is more complex than a single minimum of energy state densities. For example, as shown in FIG. 2(a), the conduction band edge 24 may include a second "valley" 28 displaced from the minimum 25. When the density of electrons sufficiently fills the energy states at the minimum conduction band edge energy 25, some of the carriers may overflow and begin to fill the lowest energy states in the nearby conduction band edge valley 28. Those overflowing electrons cannot participate in laser oscillation because they require phonon interactions in recombining with holes in the valence band. Therefore, current supplied to a semiconductor laser that results in filling of the valley 28 near the conduction band edge minimum energy 25 does not result in laser oscillation and reduces the efficiency of the conversion of electrical current to laser light. One of the external effects of that reduced efficiency is an apparent increase in the threshold current density at which laser oscillation begins.
An example of the structures of a known semiconductor laser 300 producing visible light and employing materials of the InGaAilP material system is illustrated in FIG. 1. That semiconductor laser is described in Applied Physics Letters, Volume 56, Number 18, pages 1718-1719, (1990). Referring to FIG. 1, the laser 300 includes an n-type GaAs substrate 11, a one micron thick silicon doped n-type In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P first cladding layer 12, a forty nanometer thick undoped In.sub.0.5 Ga.sub.0.5 P active layer 13, and a one micron thick zinc doped p-type In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P second cladding layer 14. Those three layers are sequentially disposed on the substrate 11 and the second cladding layer 14 includes a central mesa 19. A fifty nanometer thick zinc doped p-type In.sub.0.5 Ga.sub.0.5 P cap layer 15 is disposed on the top surface of the mesa of the second cladding layer 14. As well understood in the art, in manufacturing the semiconductor laser 300, the second cladding layer 14 and the cap layer 15 are sequentially grown and the mesa is formed by masking parts of those layers and removing the unmasked parts of the layers by etching. A zinc doped p-type GaAs contact layer 16 is disposed on and contacts the second cladding layer 14 and the cap layer 15. The thickest part of the contact layer 16 is three microns thick. The semiconductor laser 300 also includes electrodes 17 and 18 on the contact layer 16 and the substrate 11, respectively.
As in conventional semiconductor lasers, when the semiconductor laser 300 is forward biased, holes move from the contact layer 16 toward the active layer 13 and electrons move from the substrate 11 toward the active layer 13. The holes and electrons are injected into the active layer 13 where they recombine to produce light. Because the material of the first and second cladding layers is different from the material of the active layer, heterojunctions are formed at the interfaces of the active layer 13 with the first and second cladding layers 12 and 14. The heterojunctions include potential barriers that assist in confining the charge carriers injected into the active layer 13 to an active region to stimulate recombination and the emission of light. Because of the differences in the composition of the first and second cladding layers and the active layer, those layers have different refractive indices. The first refractive index of the first and second cladding layers also assists in confining light produced by carrier recombination to the active layer, defining a resonant cavity for supporting laser oscillation.
In this structure, because of the composition of the active layer 13, the laser light produced has a wavelength of about six hundred seventy nanometers. The light produced by the charge carrier recombinations and confined to the active region resonates between opposed facets 20 and 21 of the laser that are transverse to the mesa 19, resulting in laser oscillation. Most of that laser light is produced along the direction of the mesa 19 because the holes preferentially flow through the mesa toward the active layer 13. In the semiconductor laser 300, since all the materials are selected from the InGaAiP system, partially to ensure relatively close matching between the lattice constants of the different materials, the maximum difference between the energy band gap of the cladding layers and of the active layer is about 0.2 eV.
Although the InGaAlP system of materials offers the advantage of small lattice constant mismatches between various materials within that system, simplifying epitaxial growth processes, a limited amount of strain produced by lattice mismatching between contiguous layers of a semiconductor laser can have a beneficial effect, as illustrated in FIGS. 2(a) and 2(b). As already described, the typical energy state distribution as a function of wave vector for a compound semiconductor material is illustrated in FIG. 2(a). In FIG. 2(a), the energy band edges are illustrated without the presence of any stress, for example, introduced by confining the semiconductor material as a layer between two other layers having different lattice constants. That stress produces a strain, locally altering the lattice constant of the semiconductor layer as compared to the unstressed lattice constant. In FIG. 2(b), the same energy band structure for the same material as in FIG. 2(a) is shown when that material is subjected to a moderate stress. By comparing FIGS. 2(a) and 2(b), it can be seen that the stress causes the valence band edge 27 to become more parabolic, i.e., the density of energy states becomes more closely concentrated around the y axis, the wave vector value at which direct transitions of combining holes and electrons occur. As a result of this change in the distribution of the energy states, laser oscillation efficiency is improved and the threshold current is reduced.
In the InGaAlP material system, it may be desired to introduce stress to achieve improved efficiency, applying the principle illustrated in FIGS. 2(a) and 2(b). For example, the cladding layers might be AIGaP, the material within that system of materials having the largest energy band gap. Theoretically, that material would provide improved charge carrier confinement in the active layer because of increased potential barriers at the heterojunctions with the active layer as well as introducing stress because of the different lattice constants of AIGaP and of the In.sub.0.5 Ga.sub.0.5 P active layer. However, because the difference in the lattice constants of AIGaP and In.sub.0.5 Ga.sub.0.5 P is so large, the resulting stress can produce crystalline dislocations within the active layer. Dislocations can be avoided for a particular stress by making the active layer sufficiently thin. For example, when the active layer is less than ten nanometers thick, a relatively large amount of stress can be tolerated without the appearance of dislocations. However, such a thin active layer causes rapid filling of the energy states at the minimum of the conduction band with electrons so that electrons easily overflow into an adjacent valley of the conduction band edge. Thus, the improved efficiency achieved by introducing stress is lost. Therefore, suitable materials cannot be selected from the InGaAlP system for constructing a semiconductor laser producing visible light with improved efficiency. The desirable effects of stress on the active layer require that the active layer be so thin that other losses, reducing efficiency, would be experienced.